Light emitting diodes (LEDs) are becoming increasingly popular as energy-saving substitute for incandescent lamps in various applications. Unlike incandescent lamps LEDs are current-driven components and as such require driver circuits including a load current regulation. In order to reduce power dissipation within the driver circuits switched mode power supplies are usually employed to supply a LED or a series circuit of several LEDs (also referred to as LED chain) with a well-defined load current. Generally, the resulting luminous intensity (usually measured in candela) is directly proportional to the load current. The power dissipation within the driver circuit (even when including a switching converter) may, however, still become a problem which—if no security mechanism is included—may result in a thermal destruction of the driver circuit, particularly of the power stages included therein. Not only the power stages of the LED driver but also the LEDs themselves are at risk to overheat.
For this purpose many LED driver devices (including an integrated driver circuit) include a sense terminal (i.e., a chip pin) to which an external temperature sensor may be attached (usually as an option). For example, the high power white LED driver STCF02 of STM (see STMicroelectronics, data sheet STCF02, February 2007) provides a chip pin for connecting an NTC temperature sensor which is a temperature dependent resistor (thermistor) having a negative temperature coefficient (NTC). The external temperature sensor is usually used to trigger a shut-down of the device when a critical temperature has been detected.
However, in security relevant applications (e.g., the illumination of emergency exits, escape routes, emergency shut-down switches, etc.) a simple shut-down of the LED driver is insufficient as maintaining the illumination is essential. Furthermore, also in non-security related applications reliability (even in hot environments or where sufficient cooling is problematic) may also be a desired feature of an illumination device including a LED driver and respective LEDs. Finally, it is desirable to reduce the required external components necessary to operate the LED driver and to protect the driver as well as the LEDs. The still required external components should be inexpensive and easy in integrate into an illumination device.
Thus there is a need for improved LED driver circuits that are easy to use and include an intelligent over-temperature protection.